This invention relates to a separator for an alkaline battery and, more particularly for a separator for a zinc-based battery.
Separators are a crucial component in alkaline batteries. They keep the positive and negative sides of the battery separate while letting certain ions pass through while blocking other ions. The separator is a passive element that has to perform the same task unchanged for the life of the battery while withstanding a strongly alkaline environment at ambient and elevated temperatures.
For an alkaline battery, a separator should conduct hydroxyl ions at a rate sufficient to meet the high current demands of modern electronics. Films of cellulose in the form of regenerated cellulose have been used since World War II as the separator of choice for this purpose because of the superior ability of cellulose to conduct hydroxyl ion in strongly alkaline media. The low electrical resistance of 10 milliohm-in2 has also contributed to the favor among battery manufacturers for use of cellulose separators in zinc-based batteries, such as silver-zinc, zinc-nickel, and zinc manganese dioxide batteries. Additionally, the separator acts as a physical barrier to migration of other ions in the battery, such as zincate ions and silver ions in a silver-zinc battery.
In the presence of a silver cathode, regenerated cellulose performs a sacrificial role as silver metal is deposited on the separator. This can have a deleterious effect on both the water transport and the ionic conductivity of the membrane.
Limited developments have occurred in the improvement of regenerated cellulose as a battery separator. Regenerated cellulose films are the result of processing of cellulose by a xanthation reaction. Cellophane is manufactured from cellulose having a degree of polymerization between 350 and 500. There have been different attempts to treat the surface of cellophane chemically. In U.S. Pat. No. 5,763,557 Sanduja et al graft a polymer on the surface of a film of cellophane by contacting the film with a solution of silver nitrate and an alkali-metal hydroxide, then contacting a polymerizable monomer and a catalyst, the polymerization taking place directly on the surface molecules of the substrate. Similar techniques are used in U.S. Pat. No. 5,342,659. There is no evidence, however, that surface treatment of cellophane significantly retards silver deposition on the membrane.
U.S. Pat. No. 4,272,470 discloses crosslinked polyvinyl alcohol as a material that is resistant to degradation and suitable for use as a separator for an alkaline battery. The claim is that mixing a film-forming polyvinyl alcohol resin with a polyaldehyde-polysaccharide crosslinking agent will confer resistance against degradation.
Yet another approach has been to integrate wettability onto a film that would otherwise be hydrophobic. Numerous examples abound in this area. For example, U.S. Pat. No. 4,253,927 discloses modifying polyethylene film by grafting carboxyl groups onto the surface of the polyethylene film. The carboxyl groups act as carriers for ionic charge through the separator material. Degradation resistance is provided as long as the degree of grafting is a small fraction of the bulk polymer.
Cellulose, however, provides superior ionic transport to polyolefin films. It would be desirable, therefore, to manufacture a cellulose film that possesses the ability to resist silver deposition while maintaining high ionic transport.
The use of metal fluorides as agents against cellulose degradation has seen little use in alkaline batteries. U.S. Pat. No. 5,681,672 by Lee teaches adding barium fluoride to the electrolyte solution as a remedy for zinc dendrites while Adler et al in U.S. Pat. No. 5,302,475 uses an electrolyte containing KOH and a combination of KF and K2CO3 salts to inhibit zinc dendrite formation.
The separator of the present invention retards deposition of silver on its surface. The battery separator of the invention comprises cellulose, optionally a hydrogen permeable hydrophobic polymer, and an inorganic salt, preferably an insoluble fluoride salt that slowly releases from the separator to retard deposition of silver. The fluoride salt should have solubility within the range of 10 xcexcg/ml to 10 mg/ml and be in a concentration range of 2 to 20% weight of the separator. This separator when placed against the cathode of a silver oxide battery retards silver deposition on the surface of the separator.
Regenerated cellulose films of the invention are resistant to silver deposition. Performance is improved by embedding a slowly dissolving inorganic salt containing fluoride anions within the separator film. The salt purposefully and controllably leaches out of the separator at a small enough rate so as to solubilize silver ions and prevent their deposition on the separator.
These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.